The 8mm and the 7-14 zoom are the best wide angle options for the OMD. Both will work great for whale sharks. I have both, and both are very enjoyable to work with. The 8mm FE is a bit wider of a lens. The 7-14 has the added advantage of being a useful topside lens. I don't get much use from my 8mm topside.

I think the best way to decide which one is best is to look at a bunch of pictures from fish eye lenses vs rectilinear lenses and see which perspective you like the best. I wouldn't worry about trying to find pics specifically taken by these lenses to make your comparison - I would just decide which perspective through the lens you enjoy the most. Both lenses are plenty sharp.

For your stated purpose of taking pics of whale sharks, though, you can't go wrong with either lens.

Wide angle rectilinear lenses like the 7-14mm produce serious distortion, particularly in the corners where the fabric of the universe gets stretched very significantly. Fisheye lenses don't suffer from this type of distortion, but vertical and horizontal lines are curved as one moves away from the centre. Before digital, these were really the only choices. But now there is software that corrects both wide-angle rectilinear lenses and fisheye lenses in ways that eliminates bent verticals and stretching as one moves away from the centre. These corrections are not severe and so image quality is excellent post correction. For fisheye-lenses there is:

But it's only occasionally i see underwater images that i object to because of such distortions (so correction is seldom needed, but available.)

Because of this, it's more a field-of-view question about which lens to use and the advantages of having a zoom underwater when you can't change lenses. The 8mm fisheye is much wider than the wide end of the 7-14mm, and you can also use the 12mm f/2.0 behind the 4.33" dome for the 8mm (or the 3.5" dome w/extension). It's the option i've chosen, because it's smaller for travel and because above water i prefer to use faster primes.

HobieGuy, i was trying to say that you could use the 4.33" dome with both the 8mm FE and 12mm f/2.0 or use the 3.5" dome with the 8mm FE and use the 3.5" dome + extension with the 12mm f/2.0. I'm using the 4.33" dome.